It would be hard to explain crypto-currencies for kids but for teenagers can be considerable, but I can probably simplify it so they can understand what cryptocurrency is. So this is it with the difference between the real money and the crypto-currency:

Cryptocurrency
                       - this is a type of money that is digital, that can be used to trade for goods and services (obviously).
                       - this is decentralized and that means it is not controlled by a person or a group.
                       - can be considered as the "money of the internet universe or cyberspace".
                       - can be potentially stored anywhere just like credits.
                      
Real Money
                       - this is a type of money that is physically present
                       - centralized that means controlled by the government and the bank.
                       - also called as fiat currency or the typical money of the people of course
                       - can be also stored in an exchange to credits.




EDIT: If I have something missing or wrong, you can add up and/or correct me.